I-MR-R/S (Between/Within) Chart

Graphs - S Chart

The S chart assesses whether the within-subgroup variation is in control. The S chart consists of the following:

· Plotted points, which represent subgroup standard deviations, and provide an indication of the variation present within the subgroup.

· Center line (green), which is the process standard deviation (average of all the subgroup standard deviations).

· Control limits (red), which are located above and below the center line, and provide a visual means for assessing when the process is out of control. The control limits, which represent the amount of variation expected for the subgroup standard deviations, are calculated using the variation within subgroups.

Minitab conducts up to four tests for special causes for the S chart, which detect points beyond the control limits and specific patterns in the data. Points that fail are marked with a red symbol and the number of the failed test. Complete results are printed in the Session window. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These points should be investigated.

Note

Whether Minitab displays an R chart or an S chart depends on the size of the subgroups. If more than 50% of the subgroups are the same size, then the most common subgroup size determines the chart. Otherwise, the largest subgroup size determines the chart. If the subgroup size is 8 or less, Minitab displays an R chart. If the subgroup size is 9 or more, Minitab displays an S chart.

Example Output

$image\imrr_1n.gif$

Interpretation

For the injection molding data, the subgroup size is five; therefore no S chart was calculated.